Locating an implanted object based on external antenna loading

ABSTRACT

In general, the invention is directed to techniques for locating an implanted object using an external antenna. The implanted object may be, for example, an internal antenna that facilitates recharging of and/or communication with an implantable medical device. An external device coupled to the external antenna drives the antenna with a plurality of waveforms. Asymmetry in the loading profile of the external antenna when it is driven by the plurality of waveforms allows the external device or another device to determine the location of the implanted object relative to the external antenna. The external device or other device may provide information to a user based on the determined location of the implanted object relative to the external antenna, such as information to help a user position the external antenna with respect to an internal antenna in embodiments in which the implanted object is an internal antenna.

This application is a divisional of application Ser. No. 11/186,388,filed Jul. 20, 2005, which claims the benefit of U.S. ProvisionalApplication No. 60/589,538, filed Jul. 20, 2004, the entire contents ofboth of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The invention relates to techniques for aligning an external object,such as a primary antenna, with an implanted object, such as a secondaryantenna.

BACKGROUND

Many implantable medical device systems use magnetically coupledantennas for one or both of telemetry communications and inductiverecharging of a power source of the implantable medical device. In bothscenarios, the relative positioning of the primary and secondary, i.e.,external and internal, antennas is important to the performance of thesystem. This is especially true for recharging, where the positioning ofthe primary antenna has a dramatic impact on the efficiency of therecharge procedure and on the associated time that must be spentrecharging. Localization of an external device relative to an implantedobject can also be very important in, for example, implantable drug pumpsystems, where it may be necessary to locate a subcutaneous refill port.

Some existing implantable medical device systems incorporate an antenna,or other implanted object, locating feature. This feature has variouslybeen implemented using a signal strength approach, e.g., measuring thestrength of a telemetry signal sent by an implantable medical device tothe external antenna, or by using a metal detection approach, e.g.,measuring the loading of the external antenna caused by the proximity ofthe implant. However, both of these techniques are limited to providingan output proportional to the distance of separation. This output, whilesomewhat useful in guiding the user into better placement, does notcontain enough information to generate precise guidance.

Systems have also been built that locate an implanted object usingmultiple external antennas to triangulate the position of implantedobject, e.g., an internal antenna. This type of system can provide moredetailed guidance to a user for location, but only at the expense ofadditional external hardware, and additional consumption of power todrive the additional antennas.

SUMMARY

In general, the invention is directed to techniques for locating animplanted object using an external antenna. The implanted object may be,for example, an internal antenna that facilitates recharging of and/orcommunication with an implantable medical device (IMD), a refill portfor a reservoir of an IMD that includes a pump, i.e., an implantablepump, or an IMD itself. When the external antenna is driven with awaveform, induced currents in the implanted object cause loading at theexternal antenna. Asymmetry in the loading profile of the externalantenna when it is driven by different waveforms, e.g., waveforms withdifferent amplitudes, frequencies, pulse widths, or shapes, allows thelocation of the implanted object relative to the external antenna to bedetermined.

A system according to the invention may include an external devicecoupled to the external antenna. The external device may be, forexample, an external programming or recharging device for an IMD withwhich the implanted object is associated. When the user initiates animplanted object location operation, or another operation for whichlocation of an implanted object is desired, such as recharging,telemetry communication, or refilling, the external device may begindriving the external antenna with two or more waveforms in succession.For each of the waveforms, the external device measures the value of anelectrical parameter associated with the antenna, such as a currentthrough the external antenna or a voltage across the antenna, when theantenna is driven with the waveform. The external device, or anotherdevice, determines the location of the implanted object relative to theexternal antenna based on the measured electrical parameter values foreach of the waveforms.

The measured electrical parameter values differ and, due the asymmetricloading profile of the external antenna, the difference between themeasured electrical parameter values varies as a function of thelocation of the external antenna relative to the implanted object. Theexternal device or other device may apply a plurality of equations tothe measured values of the electrical parameter, each of the equationsdefining a position variable as a function of the electrical parameter.The position variables defined by the equations may include radialdistance, depth, or angle, and values thereof determined based on themeasured electrical parameter values may indicate the location of theimplanted object relative to the external antenna.

The external device or other device may provide information to a userindicating the determined location of the implanted object relative tothe external antenna. The information may include, for example, an arrowthat points in the direction of the implanted object relative to theexternal antenna, superimposed images (such as circles) that will belined up when the external antenna is aligned with the implanted object,or a text or audio description of the location of the implanted objectrelative to the external antenna. In some embodiments, such asinformation may be provided to a user as instructions, such asinstructions to help a user position the external antenna with respectto an internal antenna in embodiments in which the implanted object isan internal antenna.

The one or more equations that may be used to determine the position ofthe internal object relative to the external antenna in the mannerdescribed above may be determined prior to use of the external deviceand antenna to locate the implanted object, e.g., prior to implantationof the object, when the location of the device is known. In particular,the external device may drive the external antenna with the two or morewaveforms that will be later used during location of the object, whilethe external antenna is moved through a three-dimensional regionproximate to the object. For each of the driving waveforms, the externaldevice periodically measures value of the electrical parameter when theexternal antenna is moved through the region.

For each of the waveforms, the external device may determine arelationship between the position of the external antenna (x,y,z)relative to the internal object and the electrical parameter, based onthe measured values of the electrical parameter and the locations of theantenna when the values were measured. From these position/electricalparameter relationships, the external device may calculate equations,such as parametric equations of the form ep=a+b*z+c*z²+d*r, where ep isthe electrical parameter, e.g., current through the antenna or voltageacross the antenna, z is the depth, r is the radial distance, and a, b,c and d are coefficients calculated using known z and r values. Thevalues of r (radial distance) may be recorded as the antenna is movedthrough the three-dimensional space, or calculated afterwards based onrecorded x and y values. The external device may solve the equations foreach of the position variables as a function of the electricalparameter. For example, the external device may solve the parametricequations for r and z in terms of the electrical parameter. In thismanner, the external device may determine the plurality of equations,each equation relating a respective position variable to the electricalparameter

In some embodiments, one or more other devices, rather than the externaldevice later used to locate the implanted object, perform some or all ofthe functions associated with determination of the equations. Forexample, a robotic platform driver may move the external antenna throughthe three-dimensional region, and periodically determine the position ofthe antenna relative to the known position of the implantable object. Insuch embodiments, the platform driver or another computing device maydetermine the equations based on electrical parameter values receivedfrom the external device, and program the external device with theequations. A different external device may be used to drive the antennaand measure electrical parameter values during equation determination,and the external device that will later be used to locate the implantedobject may be programmed with determined equations and the waveforms.

Further, another device may perform some of the functions associatedwith the eventual location of the implanted object. For example, aseparate computing device may receive measured electrical parametervalues from the external device and apply the equations to the measuredvalues to determine the location of the implanted object relative to theantenna. The other device may also provide information to a user basedon the determined location, in the manner described above.

In one embodiment, the invention is directed to a method comprisingdriving an external antenna with a first waveform, measuring a firstvalue of an electrical parameter associated with the external antennawhile the external antenna is driven with the first waveform, drivingthe external antenna with a second waveform, measuring a second currentvalue of the electrical parameter while the external antenna is drivenwith the second waveform, and determining a location of an implantedobject relative to the external antenna based on the first and secondelectrical parameter values.

In another embodiment, the invention is directed to a system comprisingan external antenna, an external device coupled to the external antenna,and a processor. The external device drives the external antenna with afirst waveform, measures a first an electrical parameter associated withthe external antenna while driving the external antenna with the firstwaveform, drives the external antenna with a second waveform, andmeasures a second value of the electrical parameter while driving theexternal antenna with the second waveform. The processor determines alocation of an implanted object relative to the external antenna basedon the first and second electrical parameter values.

In another embodiment, the invention is directed to a computer-readablemedium comprising instructions. The instructions cause a programmableprocessor to control generation of a first waveform to drive an externalantenna, measure a an electrical parameter associated with the externalantenna while the external antenna is driven with the first waveform,control generation of a second waveform to drive the external antenna,measure a second value of the electrical parameter while the externalantenna is driven with the second waveform, and determine a location ofan implanted object relative to the external antenna based on the firstand second electrical parameter values.

In another embodiment, the invention is directed to a system comprisingan external antenna, an external device coupled to the external antenna,and a processor. The external device drives the external antenna withthe first waveform while the external antenna is moved through athree-dimensional region proximate to an implantable object a firsttime, periodically measures values of an electrical parameter associatedwith the external antenna while the external antenna is moved throughthe three-dimensional region the first time and driven with the firstwaveform, drives the external antenna with a second waveform while theexternal antenna is moved through the three-dimensional region a secondtime, and periodically measures values of the electrical parameter whilethe external antenna is moved through the three-dimensional region thesecond time and driven with the second waveform. The processordetermines a first position/electrical parameter relationship based onthe electrical parameter values measured while the external antenna wasdriven with the first waveform and positions within thethree-dimensional region at which the external antenna was located whenthe electrical parameter values were measured, determines a secondposition/electrical parameter relationship based on the electricalparameter values measured while the external antenna was driven with thesecond waveform and positions within the three-dimensional region atwhich the external antenna was located when the electrical parametervalues were measured, and determines first and second equations based onthe first and second position/current relationships. The first equationdefines a first position variable as a function of the electricalparameter, and the second equation defines a second position variable asa function of the electrical parameter.

In another embodiment, the invention is directed to a computer-readablemedium comprising instructions. The instructions cause a programmableprocessor to control generation of a first waveform to drive an externalantenna while the external antenna is moved through a three-dimensionalregion proximate to an implantable object a first time, periodicallymeasure values of an electrical parameter associated with the externalantenna while the external antenna is moved through thethree-dimensional region the first time and driven with the firstwaveform, determine a first position/electrical parameter relationshipbased on the electrical parameter values measured while the externalantenna was driven with the first waveform and positions within thethree-dimensional region at which the external antenna was located whenthe electrical parameter values were measured, control generation of asecond waveform to drive the external antenna while the external antennais moved through the three-dimensional region a second time,periodically measure values of the electrical parameter while theexternal antenna is moved through the three-dimensional region thesecond time and driven with the second waveform, determine a secondposition/electrical parameter relationship based on the electricalparameter values measured while the external antenna was driven with thesecond waveform and positions within the three-dimensional region atwhich the external antenna was located when the electrical parametervalues were measured, and determine first and second equations based onthe first and second position/electrical parameter relationships. Thefirst equation defines a first position variable as a function of theelectrical parameter, and the second equation defines a second positionvariable as a function of the electrical parameter.

The invention may be capable of providing one or more advantages. Forexample, systems that utilize implanted object location techniquesaccording to the invention may be able provide a user with more detailedinformation regarding the location of the object relative to theexternal antenna than is provided by systems using conventionalimplanted object location techniques. Such information may allow theuser to more quickly and accurately locate the object than is possiblewith conventional systems.

In embodiments in which the implanted object is an internal antenna, forexample, more accurate placement of the external antenna relative to theinternal antenna may improve signal strength for telemetrycommunications or coupling efficiency for recharging an IMD powersource. Improved coupling efficiency for recharging may allow shorterrecharge cycles. As another example, more accurate location of a refillport of an implantable pump may allow more accurate refilling of theimplantable pump, e.g., with fewer needle sticks.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example system for location ofan implanted object.

FIG. 2 is a conceptual diagram illustrating an example external antennathat may be used to locate an implanted object.

FIG. 3 is a flow diagram illustrating an example technique for locatingan implanted object.

FIG. 4 is a block diagram illustrating an example system for determiningposition equations that may be used to determine the location of animplanted object based on measured values of an electrical parameterassociated with an external antenna.

FIG. 5 is a flow diagram illustrating an example technique fordetermining position equations that may be used to determine thelocation of an implanted object based on measured values of anelectrical parameter associated with an external antenna.

FIG. 6A and FIG. 6B are exemplary contour maps showing one planar sliceof a three-dimensional relationship between position and current forrespective drive waveforms.

FIGS. 7A and 7B are exemplary diagrams illustrating fitting ofcurrent/position relationships with parametric equations.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating an example system 10 for locationof an implanted object. System 10 includes an external antenna 12 usedto locate an implanted object 14, and an external device 16 coupled tothe external antenna 12 by a cable or other connector 20. Externalantenna 12 may be, for example, a coil antenna.

In the illustrated embodiment, implanted object 14 is an internal, e.g.,secondary, antenna, which may also be a coil antenna, associated with animplantable medical device (IMD) 18. In such embodiments, internalantenna 14 may be used with external, e.g., primary, antenna 12 fortelemetry communication between external device 16 and IMD 18, or forrecharging of a power source of IMD 18. In such embodiments,identification of the location of internal antenna 14 relative toexternal antenna 12 may increase the accuracy of placement of externalantenna 12, e.g., alignment of antennas 12, 14, which may improve signalstrength for telemetry communications, or coupling efficiency duringrecharging.

In such embodiments, external device 16 may be a recharging device,programming device, or other interrogation device. Internal antenna 14may, as illustrated in FIG. 1, be located within a housing of IMD 18.However, internal antenna 14 may be located on the outside of thehousing of IMD 18, or be a separate structure located some distance awayfrom the housing of IMD 18 and coupled to IMD 18, e.g., via a cable.

The invention is not limited to embodiments in which implanted object 14comprises an internal antenna. For example, implanted object 14 may be afill port associated with an IMD 18 that comprises an implantablereservoir pump. In such embodiments, accurate location of a refill portmay allow more accurate refilling of the reservoir, e.g., with fewerneedle sticks. As another example, an implanted object 14 may be an IMDitself. Further, external device 16 need not be a recharging orprogramming device, but may be any external device, and used wheneverlocation of an implanted object is desired. IMD 18 may be an implantableneurostimulator, muscle stimulator, gastric stimulator, pelvic nervestimulator, bladder stimulator, cardiac pacemaker, pacemaker device withcardioversion and/or defibrillation capabilities, pump, or otherimplantable device.

In general, external device 16 exploits asymmetry in the loading profileof external antenna 12 when it is driven by a plurality of differentwaveforms to determine the location of implanted object 14 relative toexternal antenna 12. When antenna 12 is driven with a waveform, currentflows through external antenna 12, thereby generating a magnetic field.When a conductive object, such as the housing of IMD 18 or implantedantenna 14, is placed in close proximity to the coil of external antenna12, the magnetic field causes currents to flow in the conductive object.These currents in turn generate their own magnetic fields that resistthe action of the field generated by external antenna 12, which in turnaffects electrical parameters associated with the external antenna,e.g., current through or voltage across the external antenna. Thus, thevalues of such electrical parameters are related to the amount ofconductive material within the vicinity of the magnetic field ofexternal antenna.

When antenna 12 is driven with varying waveforms, such as waveforms withdifferent amplitudes, pulse widths, shapes and/or frequencies, thevalues of the electrical parameter will differ from waveform towaveform. Due the asymmetric loading profile of external antenna 12, thedifference between the measured electrical parameter values also variesas a function of the location of the external antenna relative to theimplanted object. The varying difference between the measured electricalparameter values for each of a plurality of different waveforms used todrive antenna 12 may thus be used to determine the location of animplanted object relative to external antenna 12.

When the user initiates an antenna finding operation (or an operationfor which antenna location is critical, such as recharging), such as byproviding an input via a user interface 24 of external device 16, aprocessor 22 of external device 16 controls measurement at two or moremeasurement conditions in succession. For example, processor 22 maycontrol generation of a first waveform to drive external antenna 12, andmeasure a first value of an electrical parameter associated with theantenna, e.g., current through or voltage across the antenna, while theantenna is driven with the first waveform. Processor 22 may then controlgeneration of a second waveform, different from the first, to driveexternal antenna 12, and measure a second value of the electricalparameter while the antenna is driven with the second waveform. Althoughnot shown in the Figures, external device 16 may include circuitry, suchas oscillators and amplifiers, controlled by processor 22 for generationof different waveforms. External device 16 may also include circuitryknown in the art for measuring electrical parameter values within acircuit, such as resistors and capacitors, which processor 22 may use todetermine the electrical parameter values.

Processor 22 determines a location of internal antenna 14 relative toexternal antenna 12 based on the first and second values of theelectrical parameter. In some embodiments, processor 22 applies positionequations to the measured electrical parameter values to determine thelocation of internal antenna 14 relative to external antenna 12. Each ofthe position equations defines a position variable in terms of theelectrical parameter. As examples, the following example positionequations [1] and [2] define radial distance (r) and depth (z),respectively, as a function of current through the external antenna whendriven with two different waveforms:

$\begin{matrix}{r = \frac{\left( {i_{(1)} - a - {b*z} - {c^{*}z^{2}}} \right)}{d}} & \lbrack 1\rbrack \\{z = \frac{\left( {{- B} \pm \sqrt{B^{2} - {4*A*C}}} \right)}{2*A}} & \lbrack 2\rbrack\end{matrix}$

In the equations [1] and [2] above, r is the radial distance, and z isthe depth. Further:

$\begin{matrix}{A = {v - \frac{w*c}{d}}} & \lbrack 3\rbrack \\{B = {u - \frac{w*b}{d}}} & \lbrack 4\rbrack \\{C = {t - \frac{w*a}{d} + \frac{w*i_{(1)}}{d} - i_{(2)}}} & \lbrack 5\rbrack\end{matrix}$

Values i₍₁₎ and i₍₂₎ are the currents measured when the external antennawas driven with the first and second waveforms respectively, and a, b,c, d, t, u, v and w are coefficients calculated using known depths (z)and radial distances (r). Applying equations [1] and [2] to currentvalues measured when external antenna 12 was driven with first andsecond waveforms, processor 22 may determine a radial distance betweenantennas 12, 14 and a depth of internal antenna 14 relative to externalantenna 12.

In other embodiments, the equations may define other position variablesin terms of an electrical parameter. For example, in other embodiments,processor 22 may apply equations that define radial distance and angle,instead of radial distance and depth. As shown in FIG. 1, a memory 26 ofexternal device 16 may store information describing the waveforms 28with which antenna 12 is to be driven, and the position equations 30.

Based on the determined location of internal antenna 14, processor 22may provide location information to a user to help the user locate theinternal antenna, e.g., align the external and internal antennas, viauser interface 24. For example, processor 22 may display an arrow thatpoints in the direction the user should move external antenna 12,superimposed images (such as circles) that must be aligned, a textdescription of where to move external antenna 12, or an audibledescription, via user interface 24. For example, a text description maybe output to the user identifying a radial distance from the implantablemedical device. User interface 24 may include a display, a speaker, aLight Emitting Diode (LED), or the like, and may also include any of avariety of input media, such as buttons, switches, a keypad, a touchscreen, or a pointing device.

Although the antenna positioning techniques are described in terms ofapplying two different drive waveforms, additional drive waveforms maybe used to obtain more detailed positioning. For example, addition of athird drive waveform will allow processor 22 to apply equations thatdefine radial distance (r) and angle (θ), rather than depth, in terms ofan electrical parameter. The angle (θ) may provide greater informationregarding the direction in which the internal antenna is locatedrelative to the external antenna than is known when depth (z) isdetermined.

Furthermore, the techniques of the invention can be integrated withtelemetry strength techniques for estimating distance. In particular,processor 22 may initiate telemetry communication with IMD 18 viaantennas 12, 14 to confirm the presence of IMD 18 and/or internalantenna 14.

Moreover, external antenna 12, internal antenna 14 or another implantedobject, or IMD 18 may be specifically designed to exaggerate theasymmetric loading of external antenna 12. For instance, externalantenna 12, internal antenna 14, or the housing of the implantablemedical device 18 may be an oblong shape instead of a circular shape.

Processor 22 may include a microprocessor, a controller, a digitalsignal processor (DSP), an application specific integrated circuit(ASIC), a field-programmable gate array (FPGA), discrete logiccircuitry, or the like. Memory 26 may include program instructions that,when executed by processor 22, cause processor 22 and external device 15to perform the functions ascribed to them herein. Memory 26 may includeany volatile, non-volatile, fixed, removable, magnetic, optical, orelectrical media, such as a RAM, ROM, CD-ROM, hard disk, removablemagnetic disk, memory cards or sticks, NVRAM, EEPROM, flash memory, andthe like.

FIG. 2 is a conceptual diagram illustrating an example external antenna12A that may be coupled to an external device 16 for location of animplanted object. As shown in FIG. 2, antenna includes a non-circularshape, which may increase the asymmetry of the loading of externalantenna 12 when driven with different waveforms. More particularly,antenna 12A includes two oblong portions 32A and 32B (collectively“portions 32”) that are arranged substantially similar to a cross, e.g.,are arranged substantially perpendicularly and overlap approximately attheir midpoints.

Portions 32 may be made of different materials with different loadingprofiles. Further, each of portions 32 may be driven with differentwaveforms. In this manner, the asymmetry of the loading of externalantenna 12 when driven with different waveforms may be furtherincreased.

FIG. 3 is a flow diagram illustrating an example technique for locatingan implanted object. According to the illustrated technique, externaldevice 16 drives external antenna 12 with a first waveform (34).External device 16 measures a first value of an electrical parameterassociated with external antenna 12 while the antenna is driven with thefirst waveform (36). External device 16 then drives external antenna 12with a second waveform (38), and measures a second value of theelectrical parameter (40).

External device 16 determines the location of an implanted objectrelative to external antenna 12 based on the first and second values ofthe electrical parameter (42). For example, external device 16 may applyequations that define position variables, such as radial distance (r)and depth (z) in terms of the electrical parameter, to the measuredelectrical parameter values, as described above. Based on the determinedlocation, external device 16 provides location information to aphysician or other user (44). For example, external device 16 mayprovide graphical, textual or audible indications of direction,distance, or the like, as described above.

FIG. 4 is a block diagram illustrating an example system 50 fordetermining equations that may be used to determine the location ofimplanted object 14 based on measured values of an electrical parameterassociated with external antenna 12, in the manner described above withreference to FIGS. 1 and 3. In the illustrated embodiment, system 50includes external antenna 12, antenna 14, external device 16, IMD 18 andcable 20, described above with reference to FIG. 1. In otherembodiments, system 50 may include a different external antenna,external device and cable. In other words, the external antenna,external device and cable used during determination of the equationsneed not be the same external antenna, external device and cable laterused during application of the equations to locate the internal antenna.

The one or more equations used to determine the position of internalantenna 14 relative to external antenna 12 in the manner described aboveare determined prior to use of external device 16 and antenna 12 tolocate antenna 14, when the location of antenna 14 is known. Forexample, the one or more equations may be determined prior toimplantation of IMD 18 and antenna 14.

System 50 includes a platform driver 52, e.g., a robotic platformdriver, that is capable of moving a platform 54 in a plurality ofdirections. In the illustrated embodiment, platform 54 supports antenna12, and platform driver 52 moves platform 54 such that antenna 12 ismoved through a three-dimensional region proximate to IMD 18 and antenna14. The region may be above the IMD 18 and antenna 14, or may partiallyor completely surround the antenna. In other embodiments, platform 54may support IMD 18 and antenna 14 instead of antenna 12, and platformdriver moves platform such that IMD 18 and antenna 14 are moved througha three-dimensional region proximate to antenna.

While moving platform 54, platform driver 52 may determine the positionthe platform relative to an initial position. If antennas 12, 14 arealigned prior to movement of platform 54, the position of platform 54may reflect the coordinate position (x,y,z) of antenna 12 relative toantenna 14. Platform driver 52 may periodically record such informationwhen moving platform through the region proximate to IMD 18 and antenna14. Platform driver 52 may include circuitry for communication withexternal device 16, and provide such information to the external devicefor determination of the equations.

While antenna 12 is moved through the three-dimensional region, externaldevice 16 drives the external antenna with the two or more differentwaveforms, e.g., processor 22 (FIG. 1) controls generation of two ormore waveforms to drive antenna 12. Processor 22 periodically measuresvalues of an electrical parameter for each of the waveforms whileantenna 12 is driven with the respective waveform.

Based on position information received from platform driver 52,processor 22 may determine the position of external antenna 12 relativeto internal antenna 14 when each electrical parameter value wasmeasured. Based on the measured electrical parameter values for eachwaveform, and the associated positions, processor 22 may determine arelationship between the position of external antenna 12 relative tointernal antenna 14 and the electrical parameter for each of thewaveforms. In other words, processor 22 may determineposition/electrical parameter relationships for each waveform.Position/electrical parameter relationships may take the form ofmatrices.

Processor 22 may parametrically fit the position/electrical parameterrelationships for each of the waveforms with equations of the formep=a+b*z+c*z²+d*r, where ep is the electrical parameter, e.g., currentthrough the antenna or voltage across the antenna, z is the depth, r isthe radial distance, and a, b, c and d are coefficients calculated usingknown z and r values. Platform driver 52 may record the values of r(radial distance) as antenna 12 is moved through the three-dimensionalregion, or the driver or external device may calculate radial distancesat a later time based on recorded x and y values.

Processor 22 may solve the parametric equations for each of the positionvariables as a function of the electrical parameter. For example,processor 22 may solve the parametric equations for r and z in terms ofthe electrical parameter. In this manner, the processor may determinethe plurality of equations, each equation relating a respective positionvariable to the electrical parameter.

Although described in terms of parametric equations, processor 22 mayfit the position/electrical parameter relationships with equations of ahigher order. For example, external device 16 drives external antenna 12with additional waveforms, and determines additional position/electricalparameter relationships for each of the waveforms, higher orderequations may be applied to the relationships to determine otherposition variables, such as angle (θ), as described above.

As one example, processor 22 may control generation of first and secondwaveforms to drive external antenna 12 while it is moved first andsecond times through the region by platform driver 52. For each of thewaveforms, processor 22 periodically measures the current throughantenna 12 when the antenna is driven by the respective waveform.Processor 22 may measure currents at the same times during each movementof antenna 12 through the region, i.e., when antenna 12 is located atthe same positions relative to antenna 14. Based on position informationreceived from platform driver 52 and the measured currents, processor 22determines position/current relationships for each of the waveforms.Processor 22 determines a parametric equation of best fit for each ofthe position/current relationships. For example, the parametricequations for the drive waveforms described above may be of the form:

i ₍₁₎ =a+b*z+c*z ² +d*r  [6]

i ₍₂₎ =t+u*z+v*z ² +w*r  [7]

where i₍₁₎ is current through antenna 12 when driven with the firstwaveform, i₍₂₎ is current through antenna 12 when driven with the firstwaveform, z is the depth, r is the radial distance, and a, b, c, d, t,u, v and w are coefficients calculated using known depths (z) and radialdistances (r).

Next, processor 22 solves the parametric position equations for each ofthe position variables as a function of current measurement. Forexample, the equations may be solved for r and z in terms of currentmeasurements. Below is an example in which the equations are solved forthe variable z as a function of currents i_((i)) and i₍₂₎.

$\begin{matrix}{\mspace{79mu} {r = \frac{\left( {i_{(1)} - a - {b*z} - {c^{*}z^{2}}} \right)}{d}}} & {\lbrack 1\rbrack - {{{solve}\mspace{14mu}\lbrack 6\rbrack}\mspace{14mu} {for}\mspace{14mu} r}} \\\; & {\lbrack 8\rbrack - {{{substitute}\mspace{14mu}\lbrack 1\rbrack}\mspace{14mu} {{in}\mspace{14mu}\lbrack 7\rbrack}}} \\{i_{(2)} = {t + {u*z} + {v*z^{2}} + \frac{w*i_{(1)}}{d} - \frac{w*a}{d} - \frac{w*b*z}{d} - \frac{w*c*z^{2}}{d}}} & \; \\\; & {\lbrack 9\rbrack - {{simplify}\mspace{14mu}\lbrack 8\rbrack}} \\{0 = {{\left( {v - \frac{w*c}{d}} \right)z^{2}} + {\left( {u - \frac{w*b}{d}} \right)z} + t - \frac{w*a}{d} + \frac{w*i_{(1)}}{d} - i_{(2)}}} & \; \\{\mspace{79mu} {A = {v - \frac{w*c}{d}}}} & \lbrack 3\rbrack \\{\mspace{79mu} {B = {u - \frac{w*b}{d}}}} & \lbrack 4\rbrack \\{\mspace{79mu} {C = {t - \frac{w*a}{d} + \frac{w*i_{(1)}}{d} - i_{(2)}}}} & \lbrack 5\rbrack \\{\mspace{79mu} {z = \frac{\left( {{- B} \pm \sqrt{B^{2} - {4*A*C}}} \right)}{2*A}}} & {\lbrack 2\rbrack - {{quadratic}\mspace{14mu} {equation}}}\end{matrix}$

Thus, processor 22 may determine depth (z) in by applying equation [2]to a measured current for each of the two waveforms. With z known,processor 22 may determine radial distance (r) by applying equation [1]to one of the measured currents. Equations [1] and [2] may be stored asposition equations 30 in memory 26 of external device, as illustrated inFIG. 1, along with the values for a variety of constants. Inembodiments, in which an external device different from the one used todetermine equations 30 is later used to locate internal antenna, thatexternal device may be programmed with information describing thewaveforms 28 to be used and the equations.

In some embodiments, one or more other devices, rather than externaldevice 16, perform some or all of the functions associated withdetermination of the equations. For example, platform driver 52 oranother computing device may include a processor that determines theequations based on electrical parameter values received from externaldevice, and programs the external device with the equations. Further,another device may perform some of the functions associated with theeventual location of the implanted object. For example, a separatecomputing device may receive measured electrical parameter values fromexternal device 16 and include a processor that applies the equations tothe measured values to determine the location of the implanted objectrelative to the antenna. The other device may also include a userinterface to provide information to a user based on the determinedlocation, in the manner described above with reference to externaldevice 16.

FIG. 5 is a flow diagram illustrating an exemplary initialization phasefor determining equations that define the position of external antenna12 relative to an internal object as a function of values of anelectrical parameter. External device 16 drives external antenna 12 witha first waveform of a particular frequency, shape, pulse width andamplitude (56). External antenna 12 is moved through a three-dimensionalproximate to the implanted object (58) a first time while the antenna isdriven with the first waveform, and values of the electrical parameterare periodically measured while the antenna is driven with the firstwaveform (60).

External device 16 drives external antenna 12 with a second waveform hasa different frequency, shape, amplitude, pulse width, or a combinationthereof (62). External antenna 12 is moved through the region a secondtime while the antenna is driven with the second waveform (64). Valuesof the electrical parameter are periodically measured while externalantenna 12 is moved though the region and driven with the secondwaveform (66).

Based on position information indicating the position of externalantenna 12 relative to the implanted object when the current values weremeasured, external device 16 determines relationship between position(x,y,z) and current for each of the waveforms. From these relationships,external device 16 or a separate computing device determines parabolicequations that best fit each of the position/electrical parameterrelationships (68). As described above, the equations may be of the formep=a+b*z+c*z²+d*r, where ep is the electrical parameter, e.g., currentthrough the antenna or voltage across the antenna, z is the depth, r isthe radial distance, and a, b, c and d are coefficients calculated usingknown z and r values. Using known current and position values, thevalues for the coefficients may be determined to define a parabolicequation that best fits the current/position relationships.

External device 16 or the separate computing device next determinesposition equations for z and r as a function of current measurements asdescribed in detail above (70). The position equations may be stored inexternal device 16 as position equations 30 (FIG. 1). For example, theseparate computing device may program the position equation intoexternal device 16.

FIG. 6A and FIG. 6B are exemplary contour maps showing one planar slice(z=0.5) of the three-dimensional relationship between position andcurrent for respective drive conditions. FIG. 6A illustrates a contourmap 80 showing one planar slice of the three-dimensional relationshipbetween position and current for a first drive condition, e.g., using adrive waveform with a frequency of 1115 Hz and an amplitude of 65 mV.FIG. 6B illustrates a contour map 82 showing one planar slice of thethree-dimensional relationship between position and current for a seconddrive condition, e.g., using a drive waveform with a frequency of 1095Hz and an amplitude of 55 mV. Contour maps 80, 82 illustrate asymmetricloading of antenna 12 when different waveforms are used to drive theantenna. Further, each of contour maps 80, 82 illustrates bands ofconstant current in relation to position. From such a relationship, onecan get an idea of the distance from the center of the coil of externalantenna 12 to the center of the implanted object.

FIGS. 7A and 7B are exemplary diagrams illustrating the fitting of thecurrent and position relationships with parametric equations. FIG. 7Aillustrates a parametric equation fit 84 for the first measurementcondition (i.e., drive waveform with a frequency of 1115 Hz andamplitude of 65 mV) and FIG. 7B illustrates a parametric equation fit 86for the second measurement condition (i.e., drive waveform with afrequency of 1125 Hz and amplitude of 65 mV). FIGS. 7A and 7B indicatethe form of the equation being used and identify the value of theconstants calculated using the data points collected.

Various embodiments of the invention have been described. Althoughdescribed primarily in the context of antenna alignment, the techniquesof the invention may be used in any context in which location of animplanted object is desired. For example, the techniques may be used tolocate a refill port of an implantable pump. These and other embodimentsare within the scope of the following claims.

1. A system comprising: an external antenna; an external device coupledto the external antenna that generates a first waveform, drives theexternal antenna with the first waveform while the external antenna ismoved through a three-dimensional region proximate to an implantableobject a first time, periodically measures values of an electricalparameter associated with the external antenna while the externalantenna is moved through the three-dimensional region the first time anddriven with the first waveform, generates a second waveform that has atleast one different waveform parameter than the first waveform, drivesthe external antenna with the second waveform while the external antennais moved through the three-dimensional region a second time, andperiodically measures values of the electrical parameter while theexternal antenna is moved through the three-dimensional region thesecond time and driven with the second waveform; and a processor thatdetermines a first position/electrical parameter relationship based onthe electrical parameter values measured while the external antenna wasdriven with the first waveform and positions within thethree-dimensional region at which the external antenna was located whenthe electrical parameter values were measured, determines a secondposition/electrical parameter relationship based on the electricalparameter values measured while the external antenna was driven with thesecond waveform and positions within the three-dimensional region atwhich the external antenna was located when the electrical parametervalues were measured, and determines first and second equations based onthe first and second position/electrical parameter relationships,wherein the first equation defines a first position variable as afunction of the electrical parameter and the second equation defines asecond position variable as a function of the electrical parameter. 2.The system of claim 1, wherein the first position variable comprises aradial distance, and the second position variable comprises a depth. 3.The system of claim 1, wherein the external device generates a thirdwaveform that has at least one different waveform parameter than thefirst and second waveforms, drives the external antenna with the thirdwaveform while the external antenna is moved through thethree-dimensional region a third time, and periodically measures valuesof the electrical parameter while the external antenna is moved throughthe three-dimensional region the third time and driven with the thirdwaveform, wherein the processor determines a third position/electricalparameter relationship based on the electrical parameter values measuredwhile the external antenna is driven with the third waveform andpositions within the three-dimensional region at which the externalantenna was located when the electrical parameter values were measured,and determines the first and second equations based on the first, secondand third position/electrical parameter relationships, and wherein thefirst position variable comprises a radial distance, and the secondposition variable comprises an angle.
 4. The system of claim 1, whereinthe processor determines the first and second equations by fitting aparametric equation to each of the position/electrical parameterrelationships.
 5. The system of claim 1, wherein the external antenna ismoved through the three-dimensional region proximate to the object priorto implantation of the object.
 6. The system of claim 1, wherein theprocessor comprises a processor of the external device.
 7. The system ofclaim 1, wherein the processor stores information identifying the firstand second waveforms and the first and second determined equations in amemory of the external device or of another external device fordetermination of a location of the object relative the external antennawhen the object is implanted.
 8. The system of claim 1, wherein thefirst and second waveforms are predetermined waveforms.
 9. Acomputer-readable medium comprising instructions that cause aprogrammable processor to: control generation of a first waveform in anexternal device to drive an external antenna while the external antennais moved through a three-dimensional region proximate to an implantableobject a first time; periodically measure values of an electricalparameter associated with the external antenna while the externalantenna is moved through the three-dimensional region the first time anddriven with the first waveform; determine a first position/electricalparameter relationship based on the electrical parameter values measuredwhile the external antenna was driven with the first waveform andpositions within the three-dimensional region at which the externalantenna was located when the electrical parameter values were measured;control generation of a second waveform in the external device to drivethe external antenna while the external antenna is moved through thethree-dimensional region a second time, wherein the second waveform hasat least one different waveform parameter than the first waveform;periodically measure values of the electrical parameter while theexternal antenna is moved through the three-dimensional region thesecond time and driven with the second waveform; determine a secondposition/electrical parameter relationship based on the electricalparameter values measured while the external antenna was driven with thesecond waveform and positions within the three-dimensional region atwhich the external antenna was located when the electrical parametervalues were measured; and determine first and second equations based onthe first and second position/electrical parameter relationships,wherein the first equation defines a first position variable as afunction of the electrical parameter, and the second equation defines asecond position variable as a function of the electrical parameter. 10.The computer-readable medium of claim 9, wherein the instructions thatcause a programmable processor to determine first and second equationscomprise instructions that cause a programmable processor to fit aparametric equation to each of the position/electrical parameterrelationships.
 11. The computer-readable medium of claim 9, furthercomprising instructions that cause a programmable processor to storeinformation identifying the first and second waveforms and the first andsecond determined equations in a memory of an external device that oneof programs, recharges, or refills an implantable medical deviceassociated with the implantable object.
 12. The computer-readable mediumof claim 9, wherein the instructions that cause the programmableprocessor to control the generation of the first and second waveformscomprise instructions that cause the programmable processor to controlthe generation of first and second predetermined waveforms.